Guardrails have been used for many years on our nation's highways to protected errant motorists from hazards alongside the roadway. Guardrails function by capturing errant vehicles and redirecting them away from the hazard. Hazards that are commonly protected by guardrails include trees, signs, culverts, bridge piers, steep edge drop-offs, and soft soil that could cause vehicle roll.
Guardrails are able to capture an errant vehicle by having the longitudinal strength to resist the vehicle impact. This means that the steel rail and its joints are stronger that the forces generated during the vehicle impact. The steel rail is held in place by either wood or steel posts. The posts hold the rail at the proper height and are designed to bend over and fail during an impact. These posts are individually relatively weak, however when taken as a system, they are able to resist the lateral loads imposed upon the rail. Additional structural strength is often provided to the rail by anchoring each end of the rail, either through the use of a crashworthy end terminal, or some other means of fixing the end of the steel rail to the ground.
Many end terminal designs, such as the design disclosed by Buth in U.S. Pat. No. 4,928,928, make use of an anchor cable. The anchor cable is attached on one end to the last section of guardrail and on the other end the cable is captured by a post at ground level. In this way, longitudinal forces that the rail experiences during a vehicle impact are transmitted through the cable to the ground via the post.
Although this arrangement provides an anchorage for the guardrail when it is hit downstream of the end, it also tends to make the guardrail more rigid if it is struck on the end, which may not be desirable in axial impacts. In response, some end terminal designs incorporate a breakaway post, which releases the cable if the terminal is hit on the end. Examples of posts that work in this way may be found in Buth, as well as U.S. Pat. No. 6,619,630 to Albritton, U.S. Pat. No. 5,967,497 to Denman, and US 2012/0056143 to James. The Buth and Denman devices make use of wood posts which have been weakened by placing a large hole in the center. The cable passes through and is captured by this hole. In this way the cable is restrained by the post, but if the post is broken off during an axial impact direction hit, the cable is released. Since wood has little ductility, the Buth and Denman devices fracture readily, releasing the cable, thereby reducing the possibility that the posts will retain the cable as the vehicle passes over the top.
Albritton discloses a steel break-away post that rotates around two hinge bolts during an impact. This rotation shears two frangible bolts, releasing the top half of the post, from the bottom half of the post. Although the Albritton device provides an anchorage to downstream rail impacts, while readily releasing during axial impact direction impacts, it is relatively expensive to manufacturing and requires numerous parts to be shaped and then welded together.
The device disclosed in James makes use of a single post that is modified to promote fracture. The post is modified by adding a central hole in the web and by notching the sides of the post below where the cable is attached. The James device also includes a slotted cable bracket that is welded above the central hole. Although the James device includes weakened sections to promote failure of the post, there is no guarantee that the post will actually fracture. Instead, depending upon the material used, the post may yield at the weakened sections and bend over. This is especially true if ductile materials, such as steel or aluminum are used. Indeed, James sized the central cutout of the post in such a way that the cable could pass through the post, once the post had yielded and released the cable. In this way, the James device does not rely on failure of the post to release the cable, but instead relies on the cable passing through the aperture in the post. The cable, however, may become caught on the post, rather than passing through the post, as intended.
Although adding holes and slots to posts to weaken them is well known in the art, such posts, if made of a ductile material, may tend to bend at the point of weakening, rather than break. As such, a portion of the post may remain and continue to bind the top and bottom portions of the post together.